Excessive planner time for some queries with high statistics
I'm seeing some queries, possibly to do with using a UNIQUE index,
that have fast time reported by EXPLAIN ANALYZE but the actual time as
reported by \timing at 150ms+ higher. PostgreSQL 8.4.9
Simple example queries: http://paste.ubuntu.com/726131/
Table definitions: http://paste.ubuntu.com/726193/
Rewriting the query to use common table expressions worked around the
original problem: http://paste.ubuntu.com/726141/ ('fixed' version of
the original more complex query).
We also found this problem did not occur on one of our staging
systems, which had a default statistics target of 100. Lowering the
statistics on the relavant columns from 1000 to 100 and reanalyzing
made the overhead unnoticeable.
Thoughts on IRC was this might be a regression in 8.4.9, but I haven't
got earlier versions to test with at the moment. I was asked to obtain
some traces but have not been able to organize getting tools on a
suitable server yet.
--
Stuart Bishop <stuart@stuartbishop.net>
http://www.stuartbishop.net/
Stuart Bishop <stuart@stuartbishop.net> writes:
We also found this problem did not occur on one of our staging
systems, which had a default statistics target of 100. Lowering the
statistics on the relavant columns from 1000 to 100 and reanalyzing
made the overhead unnoticeable.
eqjoinsel() is O(N^2) in the number of entries in the MCV lists.
I wouldn't expect this to be an issue unless comparison is pretty
expensive, but maybe those are string not integer columns?
Thoughts on IRC was this might be a regression in 8.4.9, but I haven't
got earlier versions to test with at the moment.
eqjoinsel has worked like that for many years. Are you claiming you
didn't see this behavior in a prior release? If so, which one?
regards, tom lane
On Sat, Nov 5, 2011 at 1:26 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
Stuart Bishop <stuart@stuartbishop.net> writes:
We also found this problem did not occur on one of our staging
systems, which had a default statistics target of 100. Lowering the
statistics on the relavant columns from 1000 to 100 and reanalyzing
made the overhead unnoticeable.eqjoinsel() is O(N^2) in the number of entries in the MCV lists.
I wouldn't expect this to be an issue unless comparison is pretty
expensive, but maybe those are string not integer columns?
No, per http://paste.ubuntu.com/726193/ there are only integer columns
being used.
Thoughts on IRC was this might be a regression in 8.4.9, but I haven't
got earlier versions to test with at the moment.eqjoinsel has worked like that for many years. Are you claiming you
didn't see this behavior in a prior release? If so, which one?
I'm repeating speculation from discussions on IRC with pg devs in my
timezone. I haven't tested with earlier versions. I can't confirm if
this problem appeared with 8.4.9 or not - it is only recently that our
query times got to the stage where we can start worrying about
milliseconds instead of seconds :-) Previously, we have never noticed
the planner overhead so had set default_statistics_target to 1000 to
minimize the chance of bad plans due to a skewed sample. 150ms+ seems
excessive though in this simple case.
--
Stuart Bishop <stuart@stuartbishop.net>
http://www.stuartbishop.net/